Freeze-drying machines (freeze-drying machines) typically do not directly cool or heat the sample, but rather achieve dehydration by freezing the sample and then removing the moisture in the form of solid water (ice). The main working principle of a freeze-drying machine includes three steps: freezing, vacuum drying, and dehydration.

1. Freezing: The sample is frozen, usually at extremely low temperatures, to freeze the moisture in it. This step transforms water from a liquid to a solid state, forming ice crystals.

2. Vacuum drying: After freezing, the sample is placed in a vacuum state. In this step, the frozen water (ice) will directly sublime, changing from a solid state to a gaseous state, bypassing the liquid phase. This process is called sublimation drying, which occurs at low temperatures (usually during freezing) rather than through heating. This is why freeze-drying machines usually do not require heating.

3. Dehydration: The sublimated water vapor is discharged from the vacuum system to ensure that all moisture in the sample is removed, leaving a dry sample.

Although freeze-drying machines themselves typically do not perform active cooling or heating operations, the process of freezing and sublimation drying involves temperature control. Therefore, freeze-drying machines need to have refrigeration and vacuum systems to maintain the required temperature and vacuum conditions. These systems are usually adjustable to ensure that the appropriate temperature is maintained throughout the freeze-drying process, in order to effectively remove moisture and maintain the stability of the sample.

In summary, freeze-drying machines achieve dehydration by freezing the sample, drying it in vacuum, and sublimating the water, usually without the need for direct cooling or heating of the sample. However, it requires precise temperature control to manage the freezing and sublimation drying processes.